Solute Diffusion in Ionic Liquids, NMR Measurements and Comparisons to Conventional Solvents

Abstract

Diffusion coefficients of a variety of dilute solutes in the series of 1-alkyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imides ([Prn1][Tf2N], n = 3, 4, 6, 8, and 10), trihexyltetracedecylphosphonium bis(trifluoromethanesulfonyl)imide [P14,666][Tf2N], and assorted imidazolium ionic liquids are measured using pulsed field gradient (1)H NMR. These data, combined with available literature data, are used to try to uncover the solute and solvent characteristics most important in determining tracer diffusion rates. Discussion is framed in terms of departures from simple hydrodynamic predictions for translational friction using the ratio ζobs/ζSE, where ζobs is the observed friction, determined from the measured diffusion coefficient D via ζobs = kBT/D, and ζSE = 6πηR is the Stokes friction on a sphere of radius R (determined from the solute van der Waals volume) in a solvent with viscosity η. In the case of neutral solutes, the primary determinant of whether hydrodynamic predictions are accurate is the relative size of solute versus solvent molecules. A single correlation, albeit with considerable scatter, is found between ζobs/ζSE and the ratio of solute-to-solvent van der Waals volumes, ζobs/ζSE = {1 + a(VU/VV)(-p)}, with constants a = 1.93 and p = 1.88. In the case of small solutes, the observed friction is over 100-fold smaller than predictions of hydrodynamic models. The dipole moment of the solute has little effect on the friction, whereas solute charge has a marked effect. For monovalent solutes of size comparable to or smaller than the solvent ions, the observed friction is comparable to or even greater than what is predicted by hydrodynamics. These general trends are shown to be quite similar to what is observed for tracer diffusion in conventional solvents.

Predicting the influence of long-range molecular interactions on macroscopic-scale diffusion by homogenization of the Smoluchowski equation

“…12 We have also assumed hydrodynamic interactions (HI) are negligible in our simulations given the small sizes assumed for the diffusing particles. 40 In the event that larger particles are considered, for which HI may be significant, 41,42 one may resort to the Rotne-Pragner HI model, 43 which yields a position-dependent diffusion tensor for which the diagonal and off-diagonal terms represent self-and inter-particle diffusivities. In principle, this tensor could be used to parameterize the spatially dependent D (y) term of the HSE, under the assumption that the obstacles are fixed relative to the diffuser.…”

Universality of Viscosity Dependence of Translational Diffusion Coefficients of Carbon Monoxide, Diphenylacetylene, and Diphenylcyclopropenone in Ionic Liquids under Various Conditions

“…44,50−52 In relation to the diffusional motion, Patra and Samanta have found that the diffusion coefficient of the solute molecule is affected by the heterogeneity of the solvation by using the fluorescence correlation spectroscopy. 53 The general trend of the relative volume dependence of the relative friction shown in Figure 6 indicates that the heterogeneous structure of ILs may not strongly contribute the diffusional motion of the solute molecule dissolved in ILs, as suggested by Kaintz et al 14 If the heterogeneous structure of ILs is a dominant factor, the dilution of [P 66614 ][NTf 2 ] with conventional liquid solvents may result in the different correlation with the case of pure ILs. To address this issue further, more detailed investigation using phosphonium or ammonium cation based ILs with different alkyl-chain lengths will be desirable together with the studies of conventional liquid solvents with longer alkyl chains.…”

“…Recently Kaintz et al reported a detailed collection of the translational diffusion coefficients in various kinds of ILs together with new data measured by PFG-NMR. 14,15 They evaluated the ratio of the diffusion coefficient determined experimentally to that estimated from the SE relation, and showed a good correlation of the ratio to the relative volume of the solute molecule to that of the solvent ions.…”

“…For the better understanding of the size effect of the diffusion coefficient, the effects of temperature and pressure should be taken into account separately, although we found that the molecular size dependence is well explained by the correlation found in the previous work. 14 The concentrations of DPCP were typically 30 mM, and the solutions were evacuated for more than 2 h before use. The solution was kept in a drybox under argon atmosphere.…”

Communication: Unusual structure and transport in ionic liquid-hexane mixtures

“…Kaintz et al reported self-diffusion of small concentrations of benzene in several ILs, including P 14,6,6,6 + /NTf − 2 . The ratio of diffusivities for C 6 H 6 to that for P 14,6,6,6 + was 28.2 at 298 K. 24 It is interesting to compare the values of the Arrhenius activation enthalpy for the shear viscosity E a,visc with the values of the self-diffusion coefficients for n-C 6 D 14 , P 14,6,6,6 + , and NTf − 2 obtained from PG-SE experiments, E a, PG−S E . We have done this for x C 6 D 14 = 0.5 to find that the values of E a,visc = 31.8 kJ/mol are quite similar to E a, PG−S E (NTf − 2 ) = 29.5 kJ/mol.…”